DESCRIPTION (adapted from investigator's abstract and/or aims): The gastrointestinal tract of the rat undergoes dramatic maturational changes during the third week of life resulting in major changes in cellular kinetics and enzyme activities. Sucrase-alpha-dextrinase, a glycoprotein hydrolase required for final absorption of dietary carbohydrate and located at the brush border membrane surface of the enterocyte, is absent prior to the third week of life when it abruptly appears and rises rapidly, as a major distinguishing feature of postnatal intestinal maturation. Although the mechanisms regulating S-D maturation are not known, there is clear evidence that luminal nutrients are required for full maturational expression of S-D. A unique animal model developed by the applicant which is now well established will be used in combination with advanced techniques of cellular and molecular biology in order to investigate the mechanisms of nutrient-dependent S-D maturation. Initial studies will determine the effect of luminal carbohydrate on the in-vivo synthesis, assembly and degradation of S-D in the maturing intestine using techniques of intraluminal radiolabeling of the nascent S-D peptide and investigating the enzyme at various stages of its synthesis, assembly and final insertion into the brush border. These studies will determine whether the kinetics of intracellular transport, the structure of S-D precursors or the degradation of S-D are altered by luminal carbohydrates. Subsequent studies will determine whether luminal carbohydrate regulates S-D maturation by transcription or translation by measuring specific mRNA for S-D using cDNA probes developed by the applicant using polymerase chain reaction (PCR) techniques and analyzing S-D structure, intracellular assembly and post-insertional processing using glycosidase probes, 2-dimensional gel electrophoresis and amino acid sequencing of the S-D protein. In order to determine whether luminal nutrients act directly on the enterocyte, the S-D response to luminal nutrients of the intestinal absorptive cells along the crypt-villus axis and in organ culture will be studied. These studies performed during steady-state conditions will be complemented by analogous studies of acute induction of S-D using intestinal perfusion techniques developed by the applicant for use in infant animals in order to determine whether a single mechanism or combinations of mechanisms participate in the S-D response. These studies will generate new information on the mechanisms by which intraluminal nutrients regulate S-D maturation during postnatal intestinal development.